Patent Document 1 discloses a vibration-suppressing technique to calculate a vibration-suppressing torque for suppressing sprung vibrations by using, as input variables, a driving torque and a wheel speed, and to decrease a control gain when a vibration amplitude of the vibration-suppressing torque continues to be greater than or equal to a predetermined amplitude for a predetermined time length (referred to as “hunting” hereinafter).
However, the technique recited in Patent Document 1 has the following problem. A condition on the road surface's side, such as a rough road, is one of a plurality of factors that produces hunting. In this case, if the output of the control is decreased continuously for a predetermined time period after the occurrence of hunting and then returned, even though hunting is not generated by the return of the output after passage through a rough road, the decreased state of the control output is continued longer than necessary and the vibration damping control does not perform sufficiently.